An infrared proper motion study of the Orion bullets

We report the first infrared proper motion measurements of the Herbig-Haro objects in OMC-1 using a 4-yr time baseline. The [Fe II]-emitting bullets a re moving of the order of 0.08 arcsec per year, or at about 170 km s(-1). T he direction of motion is similar to that inferred from their morphology. T he proper motions of H-2-emitting wakes behind the [Fe II] bullets, and of newly found H-2 bullets, are also measured. H-2 bullets have smaller proper motion than [Fe II] bullets, while H-2 wakes with leading [Fe II] bullets appear to move at similar speeds to their associated bullets. A few instanc es of variability in the emission can be attributed to dense, stationary cl umps in the ambient cloud being overrun, setting up a reverse-oriented bull et. Differential motion between [Fe II] bullets and their trailing H-2 wake s is not observed, suggesting that these are not separating, and also that they have reached a steady-state configuration over at least 100 yr. The mo st distant bullets have, on average, larger proper motions, but are not con sistent with free expansion. Nevertheless, an impulsive, or short-lived (mu ch less than 1000 yr), duration for their origin seems likely.